A novel reactor for multi-phase photocatalysis is presented, the so-called internally illuminated monolith reactor (IIMR). In the concept of the IIMR, side light emitting fibers are placed inside the channels of a ceramic monolith, equipped with a TiO2 photocatalyst coated on the wall of each individual channel. The photonic efficiency achieved with the IIMR reactor in the selective photo-oxidation of cyclohexane is 0.062, which is lower than obtained with a top illumination slurry reactor (0.151), but higher than the efficiency of an annular slurry reactor and reactor configuration with side light fibers immersed in a TiO2 slurry, reaching a photonic efficiency of 0.008 and 0.002, respectively. The results are discussed on the basis of differences in photon flows entering the reactors, and the related magnitude of product concentrations. (c) 2007 Elsevier B.V. All rights reserved.